docs: def book(self) -> Book:
          """ GnuCash Book. Opens the book or creates an database, based on settings. """
          if not self.__book:
              # Create/open the book.
              book_uri = self.settings.database_path
              self.__book = Database(book_uri).open_book(
                  for_writing=self.__for_writing)
  
          return self.__book